HomeMy WebLinkAboutH-25 MULTI-FAMILY - FDP210008 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORT
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TABLE OF CONTENTS
SUMMARY .................................................................................................................................... 1
PURPOSE AND SCOPE OF STUDY ........................................................................................... 2
PROPOSED CONSTRUCTION .................................................................................................... 2
SITE CONDITIONS ...................................................................................................................... 2
SUBSURFACE CONDITIONS ...................................................................................................... 3
LABORATORY TESTING ............................................................................................................. 4
GEOTECHNICAL ENGINEERING CONSIDERATIONS .............................................................. 5
SITE GRADING AND EARTHWORK ........................................................................................... 6
FOUNDATION RECOMMENDATIONS ...................................................................................... 11
SEISMIC DESIGN CRITERIA ..................................................................................................... 18
FLOOR SLABS ........................................................................................................................... 19
LATERAL EARTH PRESSURES ................................................................................................ 20
UNDERDRAIN SYSTEM ............................................................................................................ 23
SURFACE DRAINAGE ............................................................................................................... 24
WATER-SOLUBLE SULFATES .................................................................................................. 25
PAVEMENT THICKNESS DESIGN ............................................................................................ 25
DESIGN AND CONSTRUCTION SUPPORT SERVICES .......................................................... 30
LIMITATIONS ............................................................................................................................. 30
FIG. 1 – LOCATION OF EXPLORATORY BORINGS
FIG. 2 – LOGS OF EXPLORATORY BORINGS
FIG. 3 – LEGEND AND NOTES
FIGS. 4 through 7 – SWELL-CONSOLIDATION TEST RESULTS
FIG. 8 – GRADATION TEST RESULTS
FIG. 9 – MOISTURE-DENSITY RELATIONSHIPS
TABLE I – SUMMARY OF LABORATORY TEST RESULTS
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SUMMARY
1. Subsurface conditions encountered in the exploratory borings generally encountered
about 6 inches of topsoil at the ground surface. Borings 1 and 2 encountered about 3 feet
of man-placed fill materials. The fill materials and/or topsoil were underlain by layers of
natural sandy lean clay and poorly graded sand and gravel. The natural overburden soils
were underlain by claystone bedrock at depths ranging from about 18 to 22 feet, which
extended to the explored depths of about 30 feet.
The existing fill generally consisted of dry to moist, brown lean clay to poorly graded sand.
The horizontal and vertical limits along with the consistency of the fill were not determined
during this study. Based on sampler penetration resistance blow counts the consistency
of the fill appeared to be highly variable, suggesting the fill was not placed under controlled
conditions.
The native cohesive soils generally consisted of dry to moist, brown to gray, lean clay to
sandy lean clay. Native granular soils consisted of moist to wet, brown to tan, fine- to
coarse-grained clayey sand to poorly-graded sand with gravel. Based on sampler
penetration resistance, the consistency of the native cohesive soils ranged from stiff to
hard to occasionally soft, and the native granular soils were generally medium dense to
very dense.
The claystone bedrock was wet, fine to medium grained, and gray. Based on sampler
penetration resistance values, the claystone bedrock was very hard.
2. Shallow foundations consisting of spread footings or PT-slabs are feasible with proper
subgrade preparation for structures of less than four stories. Straight-shaft drilled piers
should be considered for structures with four or more stories.
3. Slab-on-grade construction should be feasible at the site if at least 2 feet of properly
compacted structural fill is provided below the slab subgrade elevation. Existing fill
materials should be removed from below floor slabs and replaced with properly
conditioned and compacted material.
4. Flexible pavements for light-duty pavement areas should consist of 6 inches of full-depth
asphalt, or, alternatively, a composite pavement section consisting of 4 inches of asphalt
over 8 inches of compacted aggregate base course material. Flexible pavements for
heavy-duty pavement areas should consist of 7 inches of full-depth asphalt, or,
alternatively, a composite pavement section consisting of 4.5 inches of asphalt over 8
inches of compacted aggregate base course material.
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PURPOSE AND SCOPE OF STUDY
This report presents the results of a geotechnical engineering study performed by Kumar &
Associates for the proposed Harmony 25 Development to be constructed at the southeast corner
of the intersection of Harmony Road and Strauss Cabin Road in Fort Collins, Colorado. The
project site is shown on Fig 1. The study was conducted in accordance with the scope of work in
our Proposal No. P3 19-221 to Harmony 25, LLC dated July 16, 2019.
A field exploration program consisting of six (6) exploratory borings was conducted to obtain
information on subsurface conditions. Samples of the soils and bedrock materials obtained during
the field exploration program were tested in the laboratory to determine their classification and
engineering characteristics. The results of the field exploration and laboratory testing program
were analyzed to develop recommendations for use in design and construction of the proposed
project.
This report has been prepared to summarize the data obtained during this study and to present
our conclusions and recommendations based on the proposed construction and the subsurface
conditions encountered during this study. Design parameters and a discussion of geotechnical
engineering considerations related to construction of the proposed project are included herein.
PROPOSED CONSTRUCTION
We understand the project will consist of a total of seven (7) 3-story apartment structures and one
(1) clubhouse building with a swimming pool. Each apartment building is anticipated to contain
approximately 33 to 36 units. Paved access drives, parking lots and enclosed garage parking will
also be provided around the proposed apartment buildings.
If the proposed construction varies significantly from that described above or depicted in this
report, we should be notified to reevaluate the recommendations provided in this report.
SITE CONDITIONS
The project site is located at the southeast corner of the intersection of Harmony Road and
Strauss Cabin Road (South County Road 7). The site is bounded on the north by Harmony Road,
on the west by Strauss Cabin Road, on the east by Harmony Gardens Inc, and on the south by a
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pond. The site is vegetated with grasses, weeds and bushes. The site is relatively flat with no
discernible slope.
SUBSURFACE CONDITIONS
The current field exploration program for the project was performed on August 12, 14, and 28,
2019. Six exploratory borings were drilled to depths of approximately 30 feet below ground
surface at the approximate locations shown on Fig.1 to explore subsurface conditions and to
obtain samples for laboratory testing. Logs of the exploratory borings are presented on Fig. 2
along with a legend and explanatory notes.
The borings were advanced through the overburden soils and into the underlying bedrock, where
encountered, with 4-inch diameter, continuous-flight, solid-stem augers and logged by a
representative of Kumar & Associates, Inc. Samples of the soils and bedrock materials were
obtained with a 2-inch I.D. California liner sampler. The sampler was driven into the various strata
with blows from a 140-pound hammer falling 30 inches. This sampling procedure is similar to the
standard penetration test described by ASTM Method D1586. Penetration resistance values,
when properly evaluated, indicate the relative density or consistency of the soils. The depths at
which the samples were obtained, and the associated penetration resistance values, are shown
adjacent to the boring logs on Fig. 2.
Subsurface Soil and Bedrock Conditions: The borings generally encountered about 6 inches of
topsoil at the ground surface. Borings 1 and 2 encountered about 3 feet of man-placed fill
materials. The fill materials and/or topsoil were underlain by layers of natural sandy lean clay and
poorly graded sand and gravel. The natural overburden soils were underlain by claystone
bedrock at depths ranging from about 18 to 22 feet, which extended to the explored depths of
about 30 feet. Based upon our knowledge of the area and an aerial imagery search, it appears
that there may be pockets on the site that have much deeper fill materials.
The existing fill generally consisted of dry to moist, brown lean clay to poorly graded sand. The
horizontal and vertical limits along with the consistency of the fill were not determined during this
study. Based on sampler penetration resistance blow counts the consistency of the fill appeared
to be highly variable, suggesting the fill was not placed under controlled conditions.
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The native cohesive soils generally consisted of dry to very moist, brown to gray, lean clay to
sandy lean clay. Native granular soils consisted of moist to wet, brown to tan, fine- to coarse-
grained clayey sand to poorly-graded sand with gravel. Based on sampler penetration resistance,
the consistency of the native cohesive soils ranged from stiff to hard to occasionally soft. The
native granular soils were generally medium dense to very dense.
The claystone bedrock was moist to very moist, fine to medium grained, and gray. Based on
sampler penetration resistance values, the claystone bedrock was very hard.
Groundwater Conditions: Groundwater was encountered in all of the borings during drilling at
depths ranging from about 7 feet to 14 feet below ground surface. Stabilized groundwater levels
were measured in all borings fourteen to thirty days after drilling at depths ranging from about 7
to 8 feet. Groundwater levels may fluctuate with time and may be partially dependent upon the
water level within the pond located to the south of the project site. Additional outside sources
such as precipitation and/or irrigation schemes both on and off of the site may also contribute to
a possible fluctuating groundwater level.
LABORATORY TESTING
Samples obtained from the exploratory borings were visually classified in the laboratory by the
project engineer and samples were selected for laboratory testing. Laboratory testing included
index property tests, such as moisture content (ASTM D2216), dry unit weight, grain size analysis
(ASTM D422) and liquid and plastic limits (ASTM D4318). Swell-consolidation tests (ASTM
D4546, Method B) were conducted on several samples to determine the compressibility or swell
characteristics under loading and when submerged in water. The percentage of water-soluble
sulfates was determined in general accordance with Colorado Department of Transportation
(CDOT) CP-L 2103. The results of laboratory tests performed on selected samples obtained from
the borings are shown to the right of the logs on Fig. 2, plotted graphically on Figs. 4 to 9, and are
summarized in Table 1.
Swell-Consolidation: Swell-consolidation tests were conducted on samples of the natural clay
soils and bedrock in order to determine their compressibility and swell characteristics under
loading and when submerged in water. Each sample was prepared and placed in a confining ring
between porous discs, subjected to a surcharge pressure of 200 psf or 1,000 psf, and allowed to
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consolidate before being submerged. The sample height was monitored until deformation
practically ceased under each load increment.
Results of the swell-consolidation tests are presented on Figs. 4 through 7 as plots of the curve
of the final strain at each increment of pressure against the log of the pressure. Based on the
results of the laboratory swell-consolidation testing, the natural lean clay exhibited nil to low
consolidation potential (0.1%) under a 1,000 psf surcharge pressure. The natural clayey soils
exhibited moderate swell potential (4.9%) under a 200 psf surcharge pressure. The claystone
bedrock exhibited to low swell potential (1.8%) under a 1,000 psf surcharge pressure.
Index Properties: Samples were classified into categories of similar engineering properties in
general accordance with the Unified Soil Classification System. This system is based on index
properties, including liquid limit and plasticity index and grain size distribution. Values for moisture
content, dry density, liquid limit and plasticity index, and the percent of soil passing the U.S. No.
4 and No. 200 sieves are presented in Table I and adjacent to the corresponding sample on the
boring logs. Results of gradation tests area presented on Fig. 8.
Moisture density relationships of a bulk sample of the natural overburden soils are graphically
plotted and presented on Fig. 9.
GEOTECHNICAL ENGINEERING CONSIDERATIONS
Based on the data from the field exploration and laboratory testing programs, the primary site
considerations are the presence of variable depths of undocumented fill, near-surface moisture-
sensitive native soils, isolated to occasional soft or loose native soils, and shallow groundwater
conditions. Shallow spread footing foundations and slab-on-grade construction should be feasible
with proper subgrade preparation and raising the site grades for building pads to an appropriate
height above design groundwater levels, where necessary.
In absence of placement records, the existing fill should be considered unsuitable for support of
foundations, floor slabs, and settlement- or heave-sensitive flatwork and pavements. Subgrade
preparation in areas of existing fill should include complete removal of the existing fills in those
situations and replacement with structural fill. For areas of flatwork and pavement that may be
able to tolerate some settlement- or heave-related movement, a partial removal and replacement
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option may be considered provided the owner understands and accepts the risk of potentially
unacceptable post-construction movement.
The near-surface native cohesive soils exhibited generally low swell potential or additional
compression upon wetting, although one sample of sandy lean clay exhibited a moderate potential
for collapse. Shallow foundations and soil-supported slabs underlain by these moisture-sensitive
soils will be at risk of variable heave-related total and differential movement should these soils
experience post-construction increases in moisture content. This is particularly true of lightly
loaded floor slabs. Mitigation of the risk of post-construction movement can be accomplished by
replacing the native soils to a specific depth below the bottom of footings and floor slabs with a
zone of structural fill consisting of moisture-conditioned, non- to low-swelling on-site native soils
or non-expansive imported fill materials.
Shallow groundwater may affect excavations, site grading activities, foundation and slab support,
and pavement subgrade support. Excavations extending to or below groundwater will require
temporary dewatering to facilitate excavation, subgrade preparation and placement of compacted
fill, and permanent dewatering would be necessary where the subgrade level of floor slabs,
exterior flatwork and pavements are within 2 feet of the design groundwater level, which could be
higher than the stabilized groundwater levels shown on the boring logs on Fig. 2. Permanent
dewatering would require permitting by the Water Quality Control Division of the Colorado
Department of Public Health and Environment, which may require treatment of collected
groundwater prior to discharge.
SITE GRADING AND EARTHWORK
Ideally, existing fills should be completely removed and overexcavated areas backfilled with
compacted fill meeting the material and compaction criteria presented in this section. As
discussed in specific sections of this report, the owner may elect to partially remove and replace
existing fills in areas such as hardscape and pavements that can tolerate movement possibly in
excess of normal tolerances.
Temporary Excavations: We assume that the temporary excavations will be constructed by over-
excavating the slopes to a stable configuration where enough space is available. All excavations
should be constructed in accordance with OSHA requirements, as well as state, local and other
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applicable requirements. Site excavations will encounter existing fill, native lean clays and
granular soils, and claystone to siltstone to sandstone bedrock. The existing fill and native
granular soils will classify as OSHA Type C soils. The native lean clay soils and the bedrock will
generally classify as Type A soil, although fractured or weakly-cemented bedrock may classify as
Type B and, in some cases, Type C soils depending on the degree of fracturing and cementation
and on presence of groundwater seepage. Excavations encountering groundwater could require
much flatter side slopes than those allowed by OSHA or temporary shoring. Areas where
insufficient lateral space exists may also require temporary shoring.
Surface water runoff into the excavations can act to erode and potentially destabilize the
excavation side slopes and result in soft or excessively loose ground conditions at the base of the
excavation, and should not be allowed. Diversion berms and other measures should be used to
prevent surface water runoff into the excavations from occurring. If significant runoff into the
excavations does occur, further excavation to remove and replace the soft or loose subgrade
materials or stabilize the slopes may be required.
Excavation Dewatering: Excavations extending below groundwater should be properly
dewatered prior to and during the excavation process to help maintain the stability of the
excavation side slopes and stable subgrade conditions for foundation and slab construction.
Selection of a dewatering system should be the responsibility of the contractor. Dewatering
quantities will depend on excavation size, water table drawdown, and soil permeability. Based
on gradation test results, the native cohesive soils are anticipated to have low permeability and
the native granular soils are expected to be moderately to highly permeable. Accordingly, slight
to relatively large dewatering quantities should be anticipated at the site, depending on the depth
of the excavation and the soils encountered. We are available to provide estimates of ranges of
dewatering quantities for given excavation configurations based on soil gradation characteristics.
The construction dewatering systems should be capable of intercepting groundwater before it can
reach the face of excavation side slopes, and to maintain a groundwater level at least 2 feet below
the bottom of the excavation. Dewatering should continue until construction and associated
backfilling extends above the ground water table. Dewatering systems should also be properly
designed to prevent piping and removal of soil particles which could have damaging effects.
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Fill Material: Unless specifically modified in the other sections herein, the following recommended
material and compaction requirements are presented for fill materials on the project site. A
representative of the geotechnical engineer should evaluate the suitability of all proposed fill
materials for the project prior to placement.
1. Moisture-Stabilized Fill: Fill used for site grading and beneath exterior flatwork and
pavements that are not movement sensitive may consist of properly compacted, moisture
conditioned, on-site materials provided the swell potential of those materials when
remolded to 95% of the standard Proctor (ASTM D698) maximum dry density at optimum
moisture content and wetted under a 200 psf surcharge pressure does not exceed 2%.
2. Structural Fill: Structural fill placed beneath spread footings, floor slabs and movement
sensitive exterior flatwork and pavement should consist of on-site moisture- conditioned
native soils or an imported, low permeability, non- to low-swelling material meeting the
following requirements:
Percent Passing No. 200 Sieve Maximum 70
Liquid Limit Maximum 30
Plasticity Index Maximum 12
Imported fill source materials for structural fill not meeting the above liquid limit and
plasticity index criteria may be acceptable (provided the minimum percentage passing the
No. 200 sieve is satisfied) provided they meet the swell criteria in Item 4 below.
Evaluation of potential structural fill sources, particularly those not meeting the above
liquid limit and plasticity index criteria for imported fill materials, should include
determination of laboratory moisture-density relationships and swell-consolidation tests
on remolded samples prior to acceptance.
3. Utility Trench Backfill: Materials other than claystone excavated from the utility trenches
may be used for trench backfill above the pipe zone fill provided they do not contain
unsuitable material or particles larger than 4 inches and can be placed and compacted as
recommended herein.
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4. Material Suitability: Unless otherwise defined herein, all fill material should be a non- to
low-swelling, free of claystone, vegetation, brush, sod, trash and debris, and other
deleterious substances, and should not contain rocks or lumps having a diameter of more
than 6 inches. Unless otherwise defined herein, a structural fill material generally should
be considered non- to low-swelling if the swell potential under a 200 psf surcharge
pressure does not exceed 0.5% when a sample remolded to 95% of the standard Proctor
(ASTM D698) maximum dry density at optimum moisture content is wetted.
Compaction Requirements: We recommend the following compaction criteria be used on the
project:
1. Moisture Content: Fill materials should be compacted at moisture contents within 2
percentage points of the optimum moisture content for predominantly granular materials
and between optimum and 3 percentage points above optimum for predominantly
cohesive materials. The contractor should be aware that the clay materials, including on-
site and imported materials, may become somewhat unstable and deform under wheel
loads if placed near the upper end of the moisture range.
2. Placement and Degree of Compaction: Site grading fill and structural fill should be placed
in maximum 8-inch-thick lifts. The following compaction criteria should be followed during
construction:
Percentage of Maximum
Standard Proctor Density
Fill Location (ASTM D698)
Adjacent to Foundations .................................................................................. 95%
Beneath Spread Footing Foundations ............................................................. 98%
Beneath Retaining Wall Foundations ............................................................... 98%
Wall Backfill
Upper 8 Feet of Backfill .............................................................................. 95%
Backfill Deeper than 8 Feet ....................................................................... 98%1
Beneath Floor Slabs, Exterior Flatwork and Pavements
Fill less than 8 Feet thick ........................................................................... 95%
Fill more than 8 Feet Thick ....................................................................... 98%
Utility Trenches ............................................................................................... 95%
Landscape and Other Areas ............................................................................ 95%
1 Some difficulty could be encountered achieving adequate
compaction with small equipment to avoid exerting excessive
compaction stresses on walls.
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3. Subgrade Preparation: Prior to placing site grading fill and structural fill, the upper 12
inches of the subgrade soils at the base of the fill zone should be scarified, moisture
conditioned, and recompacted to at least 95% of the standard Proctor (ASTM D698)
maximum dry density at moisture contents between optimum and 3 percentage points
above optimum moisture content. All other areas to receive new fill not specifically
addressed herein should be scarified to a depth of at least 8 inches and recompacted to
at least 95% of the standard Proctor (ASTM D698) maximum dry density at moisture
contents recommended above. Excessive wetting and drying of excavations and
prepared subgrade areas should be avoided during construction.
Some of the on-site natural soils may have relatively high moisture contents that may be
significantly higher than the optimum moisture content. The natural soils, particularly the
natural lean clays, may need to dry to moisture contents suitable for achieving proper
compaction. Moisture conditioning to dry the on-site soils could have a negative impact
on schedule, particularly if construction is done during the winter or early spring months.
It will be important to use proper construction equipment and techniques to avoid
excessive disturbance of underlying wet and potentially soft subgrade soils. Use of low
ground pressure tracked equipment or a hydraulic excavator working from outside of the
soft areas may be necessary to avoid excessive subgrade disturbance during backfilling.
The foundation excavation should also be completely dewatered such that the foundation
construction can be completed under relatively dry conditions.
Where subgrade conditions are too soft due to high moisture contents, we recommend
stabilizing the exposed subgrade to facilitate placement and compaction of structural fill
and/or construction of the foundations. Stabilization may be accomplished by over-
excavating the subgrade soils to a depth of 6 to 12 inches below planned subgrade and
backfilling with a layer of clean crushed 2-inch minus aggregate. The actual depth of over-
excavation should be determined based on the exposed subgrade conditions. Inclusion
of a geotextile stabilization fabric or geogrid between the clean crushed aggregate and
the subgrade soils may be considered to reduce the thickness of the aggregate layer.
Deeper sub-excavated soft soil areas, if necessary, may require an initial layer of 4- to 6-
inch minus rock worked into the soft soils to establish a sufficiently firm subgrade for
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subsequent fill placement, particularly if water is present. This coarser layer would need
to be overlain by a geotextile separation material and a crushed aggregate or gravel
bedding layer.
FOUNDATION RECOMMENDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend that the buildings less than four stories in height be
founded on spread footings or post-tensioned slabs (PT-slabs) placed on suitable undisturbed
native soils or structural fill extending to undisturbed native soils. Buildings greater than three
stories in height, if considered, should be supported on straight-shaft drilled footings extending
into bedrock.
Spread Footing Foundations: The design and construction criteria presented below should be
observed for a spread footing foundation system. The construction details should be considered
when preparing project documents.
1. Footings should bear on suitable undisturbed native soils or structural fill extending to
undisturbed native soils. Existing fills or areas of loose, soft, or disturbed material
encountered within the foundation excavation should be removed and replaced with
structural fill meeting the material and placement criteria in the “Site Grading and
Earthwork” section of this report.
2. Footings supported as recommended herein should be designed for an allowable soil
bearing pressure of 2,500 psf. The allowable bearing pressure may be increased by one-
third for transient loads. Footings should also be designed for a minimum soil bearing
pressure of 250 psf.
3. Spread footings placed on native soils or compacted structural fil should have a minimum
footing width of 16 inches for continuous footings, and 24 inches for isolated pads.
4. Exterior footings and footings beneath unheated areas should be provided with adequate
soil cover above their bearing elevation for frost protection. Placement of foundations at
least 30 inches below the exterior grade is typically used in this area.
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5. Based on experience we estimate total settlement for footings designed and constructed
as discussed in this section will be 1 inch or less.
6. The lateral resistance of a spread footing will be a combination of the sliding resistance of
the footing on the foundation materials and passive earth pressure against the side of the
footing. Resistance to sliding at the bottoms of the footings can be calculated based on
an allowable coefficient of friction of 0.30. Passive pressure against the sides of the
footings can be calculated using equivalent fluid density of 185 pcf. The above values are
working values.
7. Continuous foundation walls should be reinforced top and bottom to span an unsupported
length of at least 10 feet.
8. Fill placed against the sides of the footings to resist lateral loads should consist of material
meeting the material and placement criteria for structural fill procedures in the “Site
Grading and Earthwork” section of this report.
9. Granular foundation soils should be densified with a smooth vibratory compactor prior to
placement of concrete.
10. The native fine-grained soils may pump or deform excessively under heavy construction
traffic due to the high moisture content of the soils and close proximity of the groundwater
table in portions of the site. The use of track-mounted construction equipment and other
equipment that exert lower contact pressures than pneumatic tires should be used, and
the movement of vehicles over proposed foundation areas should be restricted to help
reduce this difficulty.
11. A representative of the project geotechnical engineer should observe all footing
excavations prior to concrete placement.
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PT-Slab Foundations: We assume that PT-slab foundation design will be conducted in
accordance with the Post-Tensioning Institute’s (PTI) design approach. PTI’s current design
approach is outlined in their publication "Design of Post-Tensioned Slabs-On-Ground (Third
Edition, 2012)".
The PT-slab recommendations presented below are based on the prescriptive PTI design
procedures considering how we believe the potential differential movement characteristics of the
soils underlying the PT-slabs will correlate to the required design input values. The values
presented for design are based on guidelines in the PTI Third Edition. Please note that the PTI
procedures are intended to address differential foundation movement due to the presence of
expansive soils. In this case, the soils anticipated to underlie the slabs, including the in situ natural
soils and new structural fills, will be slightly to non-expansive, so the PTI methodology cannot be
used for the native soils or similar structural fill material that may be placed during
construction. To develop the necessary design input parameters, we assumed the non-expansive
fill consisted of a relatively low swelling clay material.
The design and construction criteria presented below should be observed for a PT-slab
foundation. The construction details should be considered when preparing project documents.
1. The slab subgrade should be prepared as recommended in the “Floor Slab” section of this
report and should be designed for a maximum allowable bearing pressure of 2,500 psf.
2. Based on the methodology in PTI’s Third Edition, the slabs should be designed using the
following criteria:
Criteria Center Lift Edge Lift
Moisture variation (em) (ft) 8.9 5.3
Differential swell (ym) (in) 0.24 0.43
The parameters used to calculate these values include a soil suction (pF) value of 3.9, a
depth to constant soil suction of 8 feet, and a Type II soil. These parameters were selected
from the PTI design manual based on assumed soil index parameters and our opinion
regarding the site's differential movement potential; they are not actual measurements or
estimates of soil suction and soil moisture distributions across the site.
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3. The exterior perimeter slab beams should have sufficient embedment for frost protection.
The down-turned edges should have a minimum of 30 inches of soil cover.
We recommend an experienced PT-slab contractor construct the slabs. Representatives of the
geotechnical and structural engineer should check the foundation excavations and tendon
positions prior to placement of concrete. Fill placement and subgrade preparation should be
observed and tested by a representative of the geotechnical engineer.
Drilled Pier Foundations: The design and construction criteria presented below should be
observed for a straight-shaft drilled pier foundation system. The construction details should be
considered when preparing project documents.
1. Piers should penetrate at least 10 feet into the bedrock and have a minimum pier length
of 20 feet.
2. Piers with a minimum bedrock penetration of 10 feet should be designed for an allowable
end-bearing pressure of 30,000 psf and a skin friction of 3,000 psf for the portion of the
pier embedded into bedrock. Uplift due to structural loadings on the piers can be resisted
by using 75% of the allowable skin friction value plus an allowance for pier weight.
3. Piers should also be designed for a minimum dead load pressure of 10,000 psf based on
pier end area only. Application of dead load pressure is the most effective way to resist
foundation movement due to swelling soils and bedrock. However, if the minimum dead
load requirement cannot be achieved and the piers are spaced as far apart as practical,
the pier length should be extended beyond the minimum bedrock penetration and
minimum length to mitigate the dead load deficit. This can be accomplished by assuming
one-half of the skin friction value given above acts in the direction to resist uplift caused
by swelling soil or bedrock near the top of the pier. The owner should be aware of an
increased potential for foundation movement if the recommended minimum dead load
pressure is not met.
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4. The lateral capacity of the piers may be analyzed using the LPILE computer program and
the parameters provided in the following table. The strength criteria provided in the table
are for use with that software application only and may not be appropriate for other usages.
Material c
(psf)ø γT ks kc ε50 Soil
Type
Granular structural fill/ Native
Granular Soils Above
Groundwater
0 34 125 90 90 ---- 1
Granular structural fill/ Native
Granular Soils Below
Groundwater
0 34 63* 60 60 ---- 1
Clay Structural Fill / Native
Soils Above Groundwater 1,000 0 110 1,000 400 0.005 2
Clay Structural Fill / Native
Soils Below Groundwater 1,000 0 53* 100 ---- 0.010 2
Bedrock 4,000 0 130 1,000 400 0.005 2
*Submerged unit weight
c Cohesion intercept (pounds per square foot)
Φ Angle of internal friction (degrees)
γT Total unit weight (pounds per cubic foot)
ks Initial static modulus of horizontal subgrade reaction (pounds per cubic inch)
kc Initial cyclic modulus of horizontal subgrade reaction (pounds per cubic inch)
ε 50 Strain at 50 percent of peak shear strength
Soil Types:
1. Sand (Reese)
2. Stiff clay without free water
5. Closely spaced piers may require appropriate reductions of the lateral and axial capacities.
Reduction in lateral load capacity may be avoided by spacing the piers at least 5 pier
diameters (center to center). For axial loading, the piers should be spaced a minimum of
3 pier diameters center to center. Piers placed closer than that indicated above should be
studied on an individual basis to determine the appropriate reduction in axial and lateral
load design parameters.
If the recommended minimum center-to-center pier spacings for lateral loading cannot be
achieved, we recommend the load-displacement curve (p-y curve) for an isolated pier be
modified for closely-spaced piers using p-multipliers to reduce all the p values on the
curve. With this approach, the computed load carrying capacity of the pier in a group is
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reduced relative to the isolated pier capacity. The modified p-y curve should then be
reentered into the LPILE software to calculate the pier deflection. The reduction in
capacity for the leading pier, the pier leading the direction of movement of the group, is
less than that for the trailing piers.
For center-to-center spacing of piers in the group in the direction of loading expressed in
multiples of the pier diameter, we recommend p-multipliers of 0.8 and 1.0 for pier spacing
of 3 and 5 diameters, respectively, for the leading row of piers, 0.4 and 0.85 for spacings
of 3 and 5 diameters, respectively, for the second row of piers, and 0.3 and 0.7 for
spacings of 3 and 5 diameters, respectively, for row 3 and higher. For loading in a direction
perpendicular to the row of piers, the p-multipliers are 1.0 for a spacing of 5 diameters,
0.8 for a spacing of 3 diameters, and 0.5 for a spacing of 1 diameter. P-multiplier values
for other pier spacing values should be determined by interpolation. It will be necessary
to determine the load distribution between the piers that attains deflection compatibility
because the leading pier carries a higher proportion of the group load and the pier cap
prevents differential movement between the piers.
6. Based on the results of our field exploration, laboratory testing, and our experience with
similar, properly constructed drilled pier foundations, we estimate pier settlement will be
low. Generally, we estimate the settlement of drilled piers will be less than 1 inch when
designed according to the criteria presented herein. The settlement of closely spaced
piers will be larger and should be studied on an individual basis.
7. Piers should be designed with additional reinforcement over their full length to resist an
un-factored net tensile force from swelling soil pressure of least 50,000 pounds. The net
tensile force is for a 1.5-foot diameter pier. For larger pier diameters, this force should be
increased in proportion to the pier diameter. If the minimum dead load requirement is not
met, the tensile force should be increased by the deficit between the required minimum
dead load and the applied dead load. Similarly, the tensile force may be reduced if the
design dead load exceeds the recommended minimum dead load.
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8. A minimum 4-inch void should be provided beneath the grade beams to concentrate pier
loadings. Absence of a void space will result in a reduction in dead load pressure, which
could result in upward movement of the foundation system. A similar void should also be
provided beneath necessary pier caps.
9. A minimum pier diameter of 18 inches is recommended to facilitate proper cleaning and
observation of the pier hole. The pier length-to-diameter ratio should not exceed 30.
10. The bottom 10 feet of bedrock penetration in all pier holes should be roughened artificially
to assist in the development of peripheral shear stress between the pier and the bedrock.
Roughening should be accomplished with a grooving tool in a pattern considered
appropriate by the geotechnical engineer. Horizontal grooves at 1 to 2-foot centers or
helical grooves with a 1 to 2-foot pitch are acceptable patterns. Care should be taken that
only the bottom 10 feet of bedrock penetration portion of the pier is roughened; roughening
in the upper portion of the pier above the bottom 10 feet of the pier could increase uplift
forces on the pier resulting from swelling bedrock. The specifications should allow the
geotechnical engineer to eliminate the requirements for pier roughening if it appears that
roughening is not beneficial. This could occur if a rough surface is provided by the drilling
process or if the presence of water and/or weakly cemented materials results in a
degradation of the pier hole during roughening.
11. Difficult drilling conditions may be experienced in very hard bedrock. The drilled shaft
contractor should mobilize equipment of sufficient size and operating condition to achieve
the required bedrock penetration. A small diameter pilot hole may be required to advance
auger drilling.
12. The presence of water in the exploratory borings indicates the use of temporary casing
and/or dewatering equipment in the pier holes will be required to be excavate through
saturated granular soils and to reduce water infiltration. Where excavation encounters
cohesive soils and bedrock the requirements for casing and dewatering equipment can
sometimes be reduced by placing concrete immediately upon cleaning and observing the
pier hole. In no case should concrete be placed in more than 3 inches of water unless an
approved tremie method is used. If water cannot be removed or prevented with the use
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of temporary casing and/or dewatering equipment prior to placement of concrete, the
tremie method should be used after the hole has been cleaned.
13. Care should be taken that the pier shafts are not oversized at the top. Mushroomed pier
tops can reduce the effective dead load pressure on the piers. Sono-Tubes or similar
forming should be used at the top of the piers, as necessary, to prevent mushrooming of
the top of the piers.
14. Pier holes should be properly cleaned prior to the placement of concrete.
15. Concrete used in the piers should be a fluid mix with sufficient slump so it will fill the void
between reinforcing steel and the pier hole. We recommend a concrete slump in the range
of 5 to 8 inches be used.
16. Concrete should be placed in piers the same day they are drilled. If water is present,
concrete should be placed immediately after the pier hole is completed. Failure to place
concrete the day of drilling will normally result in a requirement for additional bedrock
penetration.
17. A representative of the geotechnical engineer should observe pier drilling operations on a
full-time basis to assist in identification of adequate bedrock strata and monitor pier
construction procedures.
SEISMIC DESIGN CRITERIA
The soil profile is anticipated to consist of about 25 feet or less of overburden soils underlain by
relatively hard bedrock. The bedrock is considered to extend to a depth of at least 100 feet below
ground surface. Overburden consisting of new structural fills and native soils will generally
classify as International Building Code (IBC) Site Class D. The underlying bedrock generally
classifies as IBC Site Class C. Based on our experience with similar profiles, including the
presences of occasional soft and loose zones we recommend a design soil profile of IBC Site
Class D. Based on the subsurface profile, site seismicity, and the anticipated depth of ground
water, liquefaction is not a design consideration.
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FLOOR SLABS
Removal and replacement of existing fill, as well as some of the natural overburden soils, to
reduce potential movements was discussed previously in the “Geotechnical Engineering
Considerations” section of this report. We recommend providing a minimum of 2 feet of properly
compacted structural fill beneath floor slabs and exterior flatwork. Some areas with fill to greater
depths than indicated by the borings may be encountered within the proposed construction limits.
Ideally, all fill in these areas should be removed from below floor slabs and exterior flatwork. The
owner should be aware and willing to accept the risk of movements in excess of normal tolerances
associated with leaving fill extending to depths greater than 2 feet in place within these areas.
Structural fill should meet the material and placement criteria provided in the “Site Grading”
section of this report. The following measures should be taken to reduce damage which could
result from movement should the underslab materials be subjected to moisture changes.
1. Floor slabs should be separated from all bearing walls and columns with expansion joints
which allow unrestrained vertical movement.
2. Interior non-bearing partitions resting on floor slabs should be provided with slip joints (at
the bottoms / at the tops) so that, if the slabs move, the movement cannot be transmitted
to the upper structure. This detail is also important for wallboards, stairways and door
frames. Slip joints which will allow at least 1.5 inches of vertical movement are
recommended.
3. Floor slab control joints should be used to reduce damage due to shrinkage cracking.
Joint spacing is dependent on slab thickness, concrete aggregate size, and slump, and
should be consistent with recognized guidelines such as those of the Portland Cement
Association (PCA) or American Concrete Institute (ACI). We suggest joints be provided
on the order of 12 to 15 feet apart in both directions. The requirements for slab
reinforcement should be established by the designer based on experience and the
intended slab use.
4. A minimum 4-inch layer of free-draining gravel should be placed beneath the slabs. This
material should consist of minus 2-inch aggregate with less than 30% passing the No. 4
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sieve and less than 5% passing the No. 200 sieve. The granular layer will prevent capillary
water rise and reduce slab curling due to differential cure.
5. If moisture-sensitive floor coverings will be used, additional mitigation of moisture
penetration into the slabs, such as by use of a vapor barrier, may be required. If an
impervious vapor barrier membrane is used, special precautions will be required to
prevent differential curing problems which could cause the slabs to warp. A minimum 2-
inch sand layer between the concrete and the vapor barrier is sometimes used for this
purpose.
6. All fill materials for support of floor slabs should be placed and compacted according to
the criteria presented in "Site Grading." The suitability of the on-site soils for use as
underslab fill is also discussed in "Site Grading."
7. Some of the natural soil and existing fill encountered during this study is suitable for use
in compacted fills beneath floor slabs.
8. All plumbing lines should be tested before operation. Where plumbing lines enter through
the floor, a positive bond break should be provided. Flexible connections should be
provided for slab-bearing mechanical equipment.
LATERAL EARTH PRESSURES
Below-grade walls and other retaining structures should be designed for the lateral earth pressure
generated by the backfill materials, which is a function of the degree of rigidity of the retaining
structure and the type of backfill material used. Retaining structures that are laterally supported
and can be expected to undergo only a moderate amount of deflection should be designed for a
lateral earth pressure based on the following equivalent fluid densities:
On-site of imported free-draining granular backfill (< 5% passing No. 200 sieve)45 pcf
On-site or imported, silty sand ..................................................................... 55 pcf
On-site or imported, moisture-conditioned clay backfill* .............................. 65 pcf
* Swell potential less than ½%
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Cantilevered retaining structures that can be expected to deflect sufficiently to mobilize the full
active earth pressure condition should be designed for the following equivalent fluid densities:
On-site or imported free-draining granular backfill (< 5% passing No. 200 sieve)...35 pcf
On-site or Imported, non-expansive, silty or clayey sand ……………………………45 pcf
On-site or imported, moisture-conditioned clay backfill* …………………………….55 pcf
* Swell potential less than ½%
The equivalent fluid densities recommended above assume drained conditions behind the
retaining structures and a horizontal backfill surface. The buildup of water behind a retaining
structure or an upward sloping backfill surface will increase the lateral pressure imposed on the
retaining structure. All retaining structures should also be designed for appropriate surcharge
pressures such as traffic, construction materials and equipment.
The zone of backfill placed behind retaining structures to within 2 feet of the ground surface should
be sloped upward from the base of the structure at an angle no steeper than 45 degrees measured
from horizontal. To reduce surface water infiltration into the backfill, the upper 2 feet of the backfill
should consist of a relatively impervious imported soil containing at least 30% passing the No. 200
sieve, or the backfill zone should be covered by a slab or pavement structure.
If mechanically stabilized earth (MSE) retaining walls are used, the reinforced zone should generally
be backfilled with CDOT Class 1 Structure Backfill material. Backfill within the reinforced zone
should be compacted to at least 95% of the standard Proctor (ASTM D698) maximum dry density.
Care should be taken not to over compact the backfill since this could cause excessive lateral
pressure on the structure. Hand compaction procedures, if necessary, should be used to prevent
lateral pressures from exceeding the design values.
An internal angle of friction of 34 degrees and a moist unit weight of 120 pcf may be used for properly
compacted granular Structure Backfill. Higher friction angles may be used for crushed aggregate
products such as Class 6 aggregate base course or crusher fines. An internal friction angle of 24
degrees and a moist unit weight of 120 pcf should be used for the on-site soils in the retained fill
zone behind the reinforced zone. Use of claystone in the retained fill should be avoided.
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Free-standing retaining structures that can tolerate some differential movement should be designed
in accordance with the recommendations provided in the “Spread Footings” section of this report.
Adequate surface drainage should be provided, and retaining structures should include subsurface
drainage provisions to reduce the potential for saturation of the backfill and the development of
hydrostatic pressures on the structure. The buildup of water behind a retaining structure will
increase the lateral earth pressure imposed on the wall.
The drainage system should consist of a drainage zone behind conventional retaining structures
and behind the facing of an MSE wall, and a perimeter underdrain system at the heel of the backfill
zone, including reinforced fill zone of MSE walls. Drainage systems for conventional retaining
structures may consist of a free-draining granular zone or manufactured drain boards placed
adjacent to the back of the retaining structures. The drainage system could connect hydraulically
to a collection system that discharges the water away from the wall. If collection and discharge is
not ideal, the wall drainage system could discharge to the exposed face of the wall via weep holes.
Perimeter underdrain systems should be designed in general accordance with the
recommendations provided in the “Underdrain System” section of this report.
We are available to design MSE walls or to perform a design review if such retaining structures are
provided by a design-build contractor. Low-height walls no more than 3 or 4 feet high may require
only minimal design, whereas steep multi-tiered systems or high walls may require global stability
analysis as well as formal design of the wall itself. At a minimum, we should review foundation
preparation and drainage provisions. We should also observe and test wall backfill placement and
compaction.
Free-draining backfill, if used, should extend down to the top of the perimeter underdrain system.
For other backfill materials, drainage should be provided by geocomposite drainage boards affixed
to the exterior walls. The geocomposite drainage board should be hydraulically connected to the
perimeter underdrain.
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UNDERDRAIN SYSTEM
Due to the relatively shallow groundwater encountered at the site, we recommend that
incorporation of a perimeter underdrain system be considered around each building. An
underdrain system would help reduce the risk of structure damage in the event that the
groundwater level increased significantly after construction is completed.
The underdrain system should consist of drain lines extending along the perimeter of the
structures at or just below the footing bearing elevation. Where feasible, the alignment of the
underdrain system should preferably be just outside of the structure perimeter, but far enough
away that the drain doesn’t interfere with construction of drilled pier foundations (if applicable).
The drain lines should consist of minimum 4-inch-diameter, rigid, perforated PVC drain pipe
placed in the bottom of a trench excavated to a depth of at least 1 foot below the base of the
overexcavated zone. The drain pipe should be surrounded above the invert level by drainage
aggregate. Drainage aggregate used in the perimeter subdrain systems should conform to the
requirements of CDOT Class B or Class C Filter Material, and the drain pipe should be factory
slotted or otherwise perforated in accordance with graded filter criteria. Alternatively, if a filter
geotextile is used in subdrain trenches to wrap the drainage aggregate, the pipes may be covered
by free-draining gravel not meeting graded filter criteria, such as AASHTO No. 57 or No. 67
Aggregate. During design, alternative drain aggregates and filtration methods can be considered.
The perforated drain pipes themselves should not be directly wrapped in geotextile due to the
potential for clogging of the geotextile at the perforations or slots.
The base of the overexcavation should be graded to slope towards the drain lines with a minimum
slope of ½%. The overall underdrain pipe system should be sloped at a minimum slope of ½%
to an overall site subdrain collection system or to a sump or sumps where water can be removed
by pumping or gravity drainage. Sumps should be provided with alarms and/or redundant pumps
in the event the pumping equipment malfunctions. In addition, the drain lines should be provided
with appropriately spaced cleanouts for maintenance and inspection, which we recommend be
performed on a routine basis. An over-designed sump and pump capacity is desirable in the
event that groundwater or other subsurface conditions change. We also believe that standby
pump capacity and standby generators should be provided in the event of pump or energy failure.
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A conceptual detail of the type of underdrain system recommended above is shown on Fig. 11.
We are available to assist in design of the underdrain system.
SURFACE DRAINAGE
Proper surface drainage is very important for acceptable performance of structures during
construction and after the construction has been completed. Drainage recommendations
provided by local, state and national entities should be followed based on the intended use of the
structure. The following recommendations should be used as guidelines and changes should be
made only after consultation with the geotechnical engineer.
1. Excessive wetting or drying of slab subgrades should be avoided during construction.
2. Exterior backfill should be adjusted to near optimum moisture content (generally ±2% of
optimum unless indicated otherwise in the report) and compacted to at least 95% of the
standard Proctor (ASTM D698) maximum dry density.
3. The ground surface surrounding the exterior of the building and movement sensitive
exterior flatwork areas should be sloped to drain away from the structure and flatwork in
all directions. We recommend a minimum slope of 6 inches in the first 10 feet in unpaved
areas. Site drainage beyond the 10-foot zone should be designed to promote runoff and
reduce infiltration. A minimum slope of 3 inches in the first 10 feet is recommended in
paved or flatwork areas. These slopes may be changed as required for handicap access
points in accordance with the Americans with Disabilities Act.
4. To promote runoff, the upper 1 to 2 feet of the backfill adjacent to the building should be
a relatively impervious on-site soil or be covered by flatwork or a pavement structure.
5. Ponding of water should not be allowed in foundation backfill material or in a zone within
10 feet of the building or areas of movement sensitive flatwork.
6. Roof downspouts and drains should discharge well beyond the limits of all backfill or be
tight-lined to planned storm water facilities.
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7. Landscaping adjacent to the building and movement sensitive flatwork areas should be
designed to avoid irrigation requirements that would significantly increase soil moisture
and potential infiltration of water within at least ten feet of the building or flatwork areas.
Landscaping located within 10 feet of the building and movement sensitive flatwork areas
should be designed for irrigation rates that do not significantly exceed evapotranspiration
rates. Use of vegetation with low water demand and/or drip irrigation systems are
frequently used methods for limiting irrigation quantities.
Lawn sprinkler heads and landscape vegetation that requires relatively heavy irrigation
should be located at least 10 feet from the building and movement sensitive flatwork areas.
Even in other areas away from the building, it is important to provide good drainage to
promote runoff and reduce infiltration. Main pressurized zone supply lines, including those
supplying drip systems, should be located more than 10 feet from the building an
movement sensitive flatwork areas in the event leaks occur. All irrigation systems,
including zone supply lines, drip lines, and sprinkler heads should be routinely inspected
for leaks, damage, and improper operation.
WATER-SOLUBLE SULFATES
The concentrations of water-soluble sulfates measured in a sample of the on-site soils obtained
from the borings indicated 0.00%. This concentration of water-soluble sulfates represents a Class
0 severity exposure to sulfate attack on concrete exposed to these materials. The degree of
attack is based on a range of Class 0, Class 1, Class 2, and Class 3 severity exposure as
presented in ACI 201. Based on the laboratory data and our experience, we believe special sulfate
resistant cement will generally not be required for concrete exposed to the natural on-site soils.
PAVEMENT THICKNESS DESIGN
A pavement section is a layered system designed to distribute concentrated traffic loads to the
subgrade. Performance of the pavement structure is directly related to the physical properties of
the subgrade soils and traffic loadings. Soils are represented for pavement design purposes by
means of a soil support value for flexible pavements and a modulus of subgrade reaction for rigid
pavements.
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Pavement design procedures are based on strength properties of the subgrade and pavement
materials assuming stable, uniform conditions. Certain soils, such as those encountered on this
site, are potentially expansive and require additional precautions be taken to provide for adequate
pavement performance. Expansive soils are problematic only if a source of water is present. If
those soils are wetted, the resulting movements can be large and erratic. Therefore, pavement
design procedures address expansive soils only by assuming they will not become wetted.
Proper surface and subsurface drainage is essential for adequate performance of pavement on
these soils.
Subgrade Materials: Based on the results of the field exploration and laboratory testing programs,
the pavement subgrade materials at the site are anticipated to generally classify between A-1-a
and A-6 with group indices between 0 and 7 in accordance with the American Association of State
Highway and Transportation Officials (AASHTO) soil classification system. Soils classifying as
A-6 would generally be considered to provide poor subgrade support. For design purposes, a
resilient modulus value of 3,500 psi was selected for flexible pavements and a modulus of
subgrade reaction of 50 pci was selected for rigid pavements.
Design Traffic: Since anticipated traffic loading information was not available at the time of report
preparation, an equivalent 18-kip daily load application (EDLA) of 5 was assumed for automobile
and light truck traffic areas (light-duty pavement), an EDLA of 15 was assumed for combined
automobile and heavier truck traffic areas, including fire lanes (heavy-duty pavement). The
designer should verify which traffic loads are valid for the project. If higher EDLA values are
anticipated, the pavement sections presented in this report will have to be reevaluated.
Pavement Sections: The pavement thicknesses were determined in accordance with the 1993
AASHTO pavement design procedures. For flexible pavement design, initial and terminal
serviceability indices of 4.5 and 2.0, respectively, were selected, with a reliability of 85 percent for
light-duty pavement areas and 85 percent for medium-duty and heavy-duty pavement areas. If
other design parameters are preferred, we should be contacted in order to reevaluate the
recommendations presented herein.
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Based on this procedure, flexible pavements for light-duty pavement areas should consist of 6
inches of full-depth asphalt, or, alternatively, a composite pavement section consisting of 4 inches
of asphalt over 8 inches of compacted aggregate base course. Flexible pavements for heavy-
duty pavement areas should consist of 7 inches of full-depth asphalt, or, alternatively, a composite
pavement section consisting of 4.5 inches of asphalt over 8 inches of compacted aggregate base
course material.
Our experience indicates full-depth asphalt sections generally perform better on expansive
subgrades than combined asphalt/aggregate base course sections. The reasons for the better
performance of full-depth asphalt are not fully understood. However, the use of aggregate base
course provides a pervious layer above a relatively impervious subgrade. The base course can
transmit water causing changes in moisture content within the potentially expansive subgrade
materials. Variations in the subgrade moisture content can be erratic and result in erratic volume
changes which cause premature deterioration of the pavement. In addition, the thinner asphalt
surface of a combined section can more easily allow water to penetrate through cracks and
migrate through the aggregate base course. High moisture contents in the subgrade or base
course will also result in loss of strength.
In lieu of an asphalt pavement section, 6 inches of Portland cement concrete may be used, in
light-duty and heavy-duty areas. Concrete pavement should contain sawed or formed joints to ¼
of the depth of the slab at a maximum distance of 12 to 14 feet on center. Because of its rigidity
concrete pavement will be more sensitive to settlement or heave-related movements than asphalt
pavement, and prone to associated cracking and distress.
Pavement Materials: The following are recommended material and placement requirements for
pavement construction for this project site. We recommend that properties and mix designs for all
materials proposed to be used for pavements be submitted for review to the geotechnical
engineer prior to placement.
1. Aggregate Base Course: Aggregate base course (ABC) used beneath HMA pavements
should meet the material specifications for Class 6 ABC stated in the current CDOT
“Standard Specifications for Road and Bridge Construction”. The ABC should be placed
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and compacted as to 95% of the maximum dry density of the modified Proctor (AASHTO
T180) within 2 percentage points of the optimum moisture content.
2. Hot Mix Asphalt: Hot mix asphalt (HMA) materials and mix designs should meet the
applicable requirements indicated in the current CDOT “Standard Specifications for Road
and Bridge Construction”. We recommend that the HMA used for this project is designed
in accordance with the SuperPave gyratory mix design method. The mix should meet
Grading S specifications with a SuperPave gyratory design revolution (NDESIGN) of 75. A
mix meeting Grading SX specification can be used for the top lift wearing course, however,
this is optional. The mix design(s) for the HMA should use a performance grade (PG)
asphalt binder of PG 64-22. Placement and compaction of HMA should follow current
CDOT standards and specifications.
3. Portland Cement Concrete: Portland Cement Concrete (PCC) pavement should meet
Class P specifications and requirements in the current CDOT “Standard Specifications for
Road and Bridge Construction”. Rigid PCC pavements are more sensitive to distress due
to movement resulting from settlement or heave of the underlying base layer and/or
subgrade than flexible asphalt pavements. The PCC pavement should contain sawed or
formed joints to ¼ of the depth of the slab at a maximum distance of 12 to 15 feet on
center.
The above PCC pavement thicknesses are presented as un-reinforced slabs. Based on
projects with similar vehicular loading in certain areas, we recommend that dowels be
provided at transverse and longitudinal joints within the slabs located in the travel lanes of
heavily loaded vehicles, loading docks and areas where truck turning movements are
likely to be concentrated. Additionally, curbs and/or pans should be tied to the slabs. The
dowels and tie bars will help minimize the risk for differential movements between slabs
to assist in more uniformly transferring axle loads to the subgrade.
The current CDOT “Standard Specifications for Road and Bridge Construction” provides
some guidance on dowel and tie bar placement, as well as in the Standard Plans: M&S
Standards. The proper sealing and maintenance of joints to minimize the infiltration of
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surface water is critical to the performance of PCC pavement, especially if dowels and tie
bars are not installed.
Subgrade Preparation: Pavement subgrade conditions are projected to generally consist of
existing non-engineered fill and/or low to moderately expansive native clay soils. These subgrade
conditions are a problem where present beneath pavements. When subjected to increases in
moisture, non-engineered fill could result in unacceptable post-construction settlement, and
expansive soils could result in potentially excessive heave.
Ideally, existing fill should be completely removed and replaced with moisture conditioned fill. If
the risk of potentially excessive post-construction settlement is acceptable to the owner, a partial
removal and replacement option may be considered. For a partial removal option, we recommend
overexcavating the existing fill encountered at planned subgrade elevation to a depth of at least
one foot below planned subgrade elevation and backfilling with moisture-conditioned fill meeting
the criteria in the “Site Grading and Earthwork” section of this report.
The native clay soils exhibited low swell potential. In areas where the native clay soils are
exposed at pavement subgrade, the subgrade soils should be over-excavated to a depth of at
least 2 feet and replaced with moisture conditioned fill. Care should be taken to place the top 1
foot of subgrade backfill at moisture contents that are not too moist, which could result in an
unstable subgrade.
Prior to placement of compacted fill or the pavement section, the exposed subgrade should be
thoroughly scarified and well-mixed to a depth of 12 inches, adjusted to a moisture content
between optimum to 3 percentage points above optimum, and compacted to 95% of the standard
Proctor (ASTM D698) maximum dry density. The pavement subgrade should also be proofrolled
with a heavily loaded pneumatic-tired vehicle. Pavement design procedures assume a stable
subgrade. Areas that deform excessively under heavy wheel loads are not stable and should be
removed and replaced to achieve a stable subgrade prior to paving.
The owner should be aware that subexcavation and replacement will reduce but not eliminate
potential movement of pavements should moisture levels increase within the expansive soils
beneath the replacement fill.
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Drainage: The collection and diversion of surface drainage away from paved areas is extremely
important to the satisfactory performance of pavement. Drainage design should provide for the
removal of water from paved areas and prevent the wetting of the subgrade soils.
DESIGN AND CONSTRUCTION SUPPORT SERVICES
Kumar & Associates, Inc. should be retained to review the project plans and specifications for
conformance with the recommendations provided in our report. We are also available to assist
the design team in preparing specifications for geotechnical aspects of the project, and performing
additional studies if necessary, to accommodate possible changes in the proposed construction.
We recommend that Kumar & Associates, Inc. be retained to provide construction observation
and testing services to document that the intent of this report and the requirements of the plans
and specifications are being followed during construction. This will allow us to identify possible
variations in subsurface conditions from those encountered during this study and to allow us to
re-evaluate our recommendations, if needed. We will not be responsible for implementation of
the recommendations presented in this report by others, if we are not retained to provide
construction observation and testing services.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
practices in this area for exclusive use by the client for design purposes. The conclusions and
recommendations submitted in this report are based upon the data obtained from the exploratory
borings at the location indicated on Fig. 1, and the proposed type of construction. This report
may not reflect subsurface variations that occur, and the nature and extent of variations across
the site may not become evident until site grading and excavations are performed. If during
construction, fill, soil, bedrock or groundwater conditions appear to be different from those
described herein, Kumar & Associates, Inc. should be advised at once so that a re-evaluation of
the recommendations presented in this report can be made. Kumar & Associates, Inc. is not
responsible for liability associated with interpretation of subsurface data by others.
Swelling soils and bedrock occur on this site. Such soils and bedrock materials are stable at their
natural moisture content but will undergo high volume changes with changes in moisture content.
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The extent and amount of perched water beneath the building site as a result of area irrigation
and inadequate surface drainage is difficult, if not impossible, to foresee.
The recommendations presented in this report are based on current theories and experience of
our engineers on the behavior of swelling soil and bedrock materials in this area. The owner
should be aware that there is a risk in constructing a building in an expansive soil and bedrock
area. Following the recommendations given by a geotechnical engineer, careful construction
practice and prudent maintenance by the owner can, however, decrease the risk of foundation
movement due to expansive soils and bedrock.
JAG/js
Rev. by: JLB
cc: book, file
Project No.: 19-3-185Project Name: Harmony 25Date Sampled: August 14, 2019Date Received: August 22, 2019Boring Depth (Feet)Gravel (%) Sand (%)Liquid Limit (%)Plasticity (%)1 4 8/27/19 18.5 104.5 0 36 64 31 13 0 A-6 (6) Sandy Lean Clay (CL)2 4 8/22/19 7.4 123.7 0 47 53 25 7 A-4 (1) Sandy Lean Clay (CL)3 1 8/27/19 7.2 111.9 8 47 45 34 16 A-6 (4) Clayey Sand (SC)4 4 8/30/19 7.0 132.0 19 40 41 25 10 A-4 (1) Clayey Sand with Gravel (SC)4 29 8/30/19 11.7 118.7Claystone Bedrock5 1 8/22/19 2.6 120.2 18 72 10 NV NP A-1-a (0) Poorly Graded Sand with Silt and Gravel (SP-SM)5 4 8/28/19 19.2 102.7 0 39 61 32 16 A-6 (7) Sandy Lean Clay (CL)6 1 8/30/19 2.5 140.3 7 73 20 NV NP A-2-4 (0) Silty Sand (SM)4 1 to 5 9/3/19 11.7* 118.2* 14 37 49 31 17 A-6 (5) Clayey Sand (SC)Table ISample Location Gradation Atterberg LimitsDate TestedNatural Moisture Content (%)Natural Dry Density (pcf)Percent Passing No. 200 Sieve* - Optimum moisture content and maximum dry density as determined by standard Proctor (ASTM D 698)Water Soluble Sulfates (%)AASHTO Classification (Group Index) Soil or Bedrock TypeSummary of Laboratory Test Results